The present invention relates to electron multiplier devices, and more specifically relates to dynode arrays, methods of making the same and components incorporating the same.
Photomultiplier tubes (PMT) are versatile, sensitive detectors of radiant energy in the ultraviolet, visible, and near infrared regions of the electromagnetic spectrum. A photomultiplier tube consists of a photoemissive photocathode, an electron multiplier device based on secondary electron emission and an anode to collect the signal electrons, all housed inside a vacuum envelope. Radiant energy such as light incident on the photocathode causes the photocathode to emit electrons. In the electron multiplier device, these electrons are accelerated by an electric field towards an electrode referred to as a xe2x80x9cdynodexe2x80x9d. As the electrons impinge on the dynode, they cause the dynode to emit a larger number of secondary electrons which are in turn accelerated to another dynode producing more secondary electrons. This process continues for several stages, with progressively larger numbers of electrons being emitted at each successive stage. The electrons from the last dynode stage are collected on an anode which is connected to an external circuit, outside of the vacuum envelope. The dynodes may be arranged to provide a tortuous path which changes direction at each dynode. This helps to assure that electrons from each dynode will impinge on the next dynode, and also protects the photocathode against positive ions which may be emitted from the anode or from the dynodes. PMT""s are used in industrial and scientific apparatus as detectors in systems for measuring the intensity of a beam of radiant energy. For a large number of applications, the PMT is the most sensitive detector available. The superiority of the PMT arises from the secondary electron emission amplification, which makes it possible for the device to approach xe2x80x9cidealxe2x80x9d device performance limited only by the statistics of photoemission. The electron gain of a PMTxe2x80x94the ratio of the number of electrons provided by the last stage to the number of electrons provided by the photocathodexe2x80x94typically ranges from 103 to as high as 108. Thus, even when the radiant energy to be detected is extremely weak, a PMT can provide output signals at levels which are easily measured by auxiliary electronic equipment. PMTs can also have extremely fast time response (xcx9c100 ps), which provides the capability for measuring radiant energy varying at a rapid rate. Stated another way, the combination of gain and bandwidth provided by PMTs is unmatched by any other detector. PMTs have very low quiescent power when the individual dynodes are powered separate from an active power supply circuit. A dynode set can also be used to amplify a stream of electrons or ions from a source other than a photocathode, and can provide similar advantages in these applications.
The dynode sets used in measurement devices typically provide only one channel or set of cascaded dynode stages, and amplify only one stream of electrons. Thus, in a light-sensing PMT, the electrons emitted by the entire photocathode are amplified together in one stream of electrons so that the device provides a single output signal representing the light incident on the entire photocathode.
Imaging devices typically must process a separate signal for each of many picture elements or xe2x80x9cpixelsxe2x80x9d in a two-dimensional array of pixels constituting an image. For example, a monochrome (black and white) image can be represented by a set of signals, each representing the brightness of the image within a pixel at a particular position. Many common imaging devices, such as the charge-coupled-device or xe2x80x9cCCDxe2x80x9d imaging devices in home video cameras and in electronic still cameras incorporate a two-dimensional array of detectors incorporating a separate detector for each pixel in the image. A lens focuses the image onto the array, and each detector provides a signal representing the brightness of one pixel in the image. These signals can be reconstructed to provide an image, such as a television or still picture representing the original image. To provide reasonable resolution in the resulting image, the imaging device should include a large number of detectors. Even a medium-quality imaging system such as a consumer video camera requires tens of thousands of pixels; high quality imaging requires hundreds of thousands of pixels. However, with common CCD technology, there is a direct relationship between the size of each detector and the sensitivity of the device, and a similar relationship between the size of each detector and immunity to random electronic noise. Thus, the spatial resolution of the devicexe2x80x94the number of detectors which can be provided in a device of a given sizexe2x80x94is limited. CCD technology has branched into two major classes. One class provides low cost sensors for large consumer markets such as camcorders, line scanners, etc. whereas the other class provides very high quality CCDs for scientific imaging. The low cost sensors are capable of achieving high data rates (xcx9c60-100 MHz for certain line scanners), they suffer in image quality and are not satisfactory for high frame rate scanning arrays. The high quality CCD sensors, while providing excellent low noise performance, cannot provide that performance at high frame rates. Thus, while Si based CCD technology has made great progress, there is still a large gap between what is desired for high quality imaging and performance of the present generation of CCD sensors. The CCD devices do not provide the high gain, bandwidth and response time of dynode devices.
Attempts have been made to fabricate plural-channel dynode arrays heretofore. Ehrfeld et al., U.S. Pat. No. 4,990,827 and Shimabukuro et al. U.S. Pat. No. 5,329,110 propose making arrays of small electron multipliers by certain microfabrication techniques. However, the techniques and structures taught by these references are suitable for making linear arrays of electron multipliers; they are not well suited to fabrication of a two-dimensional array of dynode channels.
Comby et al., Nuc. Inst. Meth. Phys. Res. A 343, 263 describe an all ceramic multichannel electron multiplier in a PMT having four imaging pixels of 0.6 mm diameter employing a five stage dynode structure. The dynodes are provided as metallic plates arranged along a channel. Openings in the plates are offset from one another to form a tortuous path. According to the reference, the results of gain measurements from these devices demonstrated that machined channels can be built with high gain. Using a Agxe2x80x94Oxe2x80x94Cs coated dynode material, they were able to achieve gains of about 100 for the five stage multiplier, amplifying photoelectrons from a CS3Sb photocathode. As set forth in Comby et al, Proceedings. International Conference On Inorganic Scintillators and Their Applications, SCINT95, DELFT Univ. of Tech. The Netherlands, September (1995), by treating Au dynodes with Sbxe2x80x94Cs, gains in excess of 103 were demonstrated in an all ceramic PMT with 0.6 mm pixels in a 4xc3x974 array. These articles propose that it may be possible to fabricate a 256-pixel device. Thus, dynode array devices available heretofore do not provide the spatial resolution needed for high-quality imaging.
Another electron multiplying device is known as a microchannel plate or xe2x80x9cMCPxe2x80x9d. MCP""s typically have numerous continuous channels extending through an insulating layer. A coating of a material having high electron emissivity is applied on the interior of each channel. The coating has a high electrical resistance. A voltage applied through electrically conductive layers extending on opposite side of the insulating layer creates a potential gradient between opposite ends of the coating. Electrons entering each channel are accelerated along the channel by the potential gradient, and impinge on the walls of the channel. Such collisions yield secondary electrons which are also accelerated and provide further collisions. Although MCP""s provide advantages such as fast rise times, high spatial resolution and low cross-talk between adjacent channels, the gain of a MCP deteriorates at relatively low electron currents. After electrons are emitted from each portion of the electron-emissive channel lining, that part of the layer must be recharged by conduction through the lining. The high resistance of the lining limits the rate of recharging. Thus, the gain of a typical MCP deteriorates significantly at electron currents of about 0.1 Coulomb per cm2 of plate area. Moreover, because current continually flows through the resistive coatings on the channel walls, MCPs typically draw appreciable power at all times. This limits their use in battery-powered devices.
One aspect of the present invention provides a multichannel microdynode device. A device in accordance with this aspect of the invention includes a porous structure defining an entry side and an exit side. The structure incorporates a plurality of dynode layers and a plurality of electrically insulating spacer layers. These layers are disposed in alternating sequence between the entry side and the exit side. Each microchannel has an entrance aperture at the entry side of the porous structure and an exit aperture at the exit side of the porous structure. Each channel has a lengthwise direction between the entrance and exit apertures. As used herein with reference to a channel, the term xe2x80x9cforward directionxe2x80x9d means lengthwise direction from the entrance aperture to the exit aperture, whereas the xe2x80x9creversexe2x80x9d direction is the opposite direction. Each microchannel preferably has a mean diameter less than about 150 microns. The structure defines walls surrounding each microchannel and substantially segregating each microchannel from the other microchannels. The device further includes an electron emissive material in the microchannels within the dynode layers and means for connecting the dynode layers to biasing voltages. The term xe2x80x9celectron-emissive materialxe2x80x9d as used herein refers to a material having a high coefficient of secondary electron emission.
Most preferably, the dynode layers and the spacer layers are bonded to one another and form a monolithic structure. In a particularly preferred arrangement, the dynode layers and the spacer layers have confronting surfaces bonded to one another over substantially the entire extent of these surfaces other than the areas occupied by the microchannels. The confronting surfaces of the dynode layers and the spacer layers may be bonded directly to one another or else may be bonded to one another by layers of bonding material interposed between these layers. The dynode layers may be fabricated from a semiconductive or nonconductive structural materials such as undoped silicon and may have via liners formed from an electrically conductive material such as a metal overlying the structural material on the dynode regions of the microchannel walls, within the dynode layers. In this instance, the dynode layer may include, or may be contiguous with, a conductive layer such as a metallic layer which extends to the conductive walls of the holes. Alternatively, the dynode layer may be formed from a conductive material such as a metal, and the metal of the dynode layers may define the interior walls of the holes in the dynode layer. In either case, the conductive layers act to connect the dynode layers to biasing voltage and provide a direct, conductive connection to the interiors of the holes in the dynode layer. The conductive layer in or contiguous with each dynode layer may be connected to a source of voltage at a potential different from the potentials connected to the other dynode layers. Thus, a potential gradient is maintained along the length of each microchannel by the different potentials, with more positive potential toward the exit aperture. Electrons entering the entrance aperture of each microchannel will be accelerated along the channel in the forward direction and will impinge upon the dynode layers, causing secondary electron emissions. The secondary electrons in turn are accelerated and pass along the channel where they impinge upon the walls in further dynode layers, and the process continues to provide electron multiplication or gain.
In particularly preferred structures according to this aspect of the present invention, the microchannels have mean diameters less than about 100 microns, more preferably less than about 25 microns, and most preferably between about 5 microns and about 10 microns. The small diameter of the microchannels provides several significant effects. A substantial number of electrons will collide with the walls of the channel even if the channel is straight or only gently curved. Likewise, any positively charged ions entering the channels at the exit end or generated within the channels will have a high probability of collision with the walls of the channels. Accordingly, the probability of an ion being accelerated along the channel and passing out of the channel in the reverse direction will be very small. Thus, although the channel may be curved, it is not necessary to provide a tortuous path. Thus, the central axis of each microchannel may be substantially straight and may extend in a smooth curve or else may extend in a smooth curve, desirably with two or fewer changes of direction of curvature between the entry side of the structure and the exit side. The microchannels may have essentially any cross sectional shape. A cross sectional shape which is a circle or a regular polygon, such as a square, is particularly preferred. In a particularly preferred arrangement, the walls of the microchannel slope inwardly towards one another within each dynode layer in the direction through the dynode layer towards the exit end of the structure. This further enhances the probability of electron collisions with the walls of the dynode layer and hence further enhances the gain of the system. In a particularly preferred arrangement, the center-to-center distance between the central axes of adjacent microchannel ranges from about 1.01 to about 2 times the main diameter of each microchannel. Thus, the microchannels occupy a substantial portion of the area of the porous structure. Stated another way, the open area or combined cross sectional areas of the microchannels measured at the entrance apertures of the microchannels desirably constitutes at least about 50% and more preferably at least about 75% of the area of the porous structure. Still higher open area percentages, in some cases up to 98%, at the entrance apertures are attainable where the microchannels taper in the forward direction.
In a further embodiment of the invention, the dynode layers may be provided with mesh structures subdividing each microchannel at each dynode layer. Each such mesh structure has non-scale passages, substantially smaller than the microchannel, extending through it in the lengthwise direction. The walls of these passages have the electron emissive material thereon. Such a mesh structure provides an even greater probability of collisions between electrons passing lengthwise along the channel, and an even greater probability of collision for any positive ions passing in the reverse direction along the channel.
The preferred electron multiplier structures in accordance with the foregoing aspects of the invention provide all of the advantages of a dynode structure, including high gain, low current consumption and high frequency response, and can also provide very high spatial resolution. Thus, the preferred structures in accordance with the foregoing aspects of the invention provide closely spaced microchannels. Moreover, the microchannels are effectively isolated from one another, so that the structure provides low crosstalk between adjacent channels. In effect, the preferred structures in accordance with this aspect of the invention combine the best advantages of microchannel plates with the best advantages of dynode structures.
Electron multiplier structures in accordance with the foregoing aspects of the invention desirably are used in conjunction with a cathode structure capable of emitting electrons overlying the entry side of the structure so that regions of the cathode structure are exposed to entrance apertures of the microchannels, and an anode structure overlying the exit side of the porous structure. Most preferably, the cathode structure and the anode structure are sealingly connected to the porous structure, so that the anode structure, cathode structure and porous structure cooperatively maintain vacuum within the microchannels. The anode structure and cathode structure may be formed with the porous structure, or bonded to the porous structure, to provide a single monolithic device. Such a monolithic device provides a compact, rugged unit which can be employed without any external shell or vacuum envelope. The anode structure may include conductors extending to the exterior of the monolithic device, which eliminates the need for any separate feed-throughs. The cathode structure may incorporate a photocathode adapted to emit electrons in response to light, whereas the anode structure may incorporate a plurality of separate anodes overlying the exit apertures of the microchannels. Preferably, an individual anode is aligned with the exit aperture of each microchannel. In this case, the electrons impinging on the individual anode associated with each microchannel will represent light impinging on the particular region of the photocathode overlying the entry aperture of that microchannel. Thus, the device will provide a plurality of separate signals, each representing the brightness of light in a single pixel. These signals can be handled and processed in a microelectronic circuit. The microelectronic circuit may be formed as part of the same monolithic structure with the other elements of the device. Such a monolithic device can be used instead of a CCD sensor and can be made with comparable spatial resolution to a CCD sensor. However, the device in accordance with these embodiments of the invention can provide markedly superior signal output levels and bandwidth.
In other embodiments, the anode structure may incorporate a phosphor layer adapted to emit light in response to electrons impinging on the phosphor layer. Where the cathode structure includes a photocathode, the device will act as a light amplifier; the light emitted by the anode phosphor will be far brighter than that impinging on the photocathode. These devices can be incorporated in night vision systems. In still other embodiments, the cathode structure may include plural individual cathodes adapted to emit electron currents. An appropriate circuit may be provided for selectively energizing individual cathodes to cause these individual cathodes to emit. This will cause individual portions of the anode structure phosphor layer to be illuminated. Such a device may be used as a flat panel display.
Further aspects of the present invention provide methods of making microdynode devices. A method in accordance with one embodiment of the invention includes the steps of providing a plurality of electrically insulating spacer layers having holes therein and providing a plurality of dynode layers also having holes therein. These steps are performed so that the spacer layers and dynode layers are stacked in alternating sequence, with at least one of the dynode layers being sandwiched between two of the spacer layers and so that the holes in the dynode layers are aligned with the holes in the spacer layers to form continuous microchannels extending through the stack. A method in accordance with this aspect of the present invention desirably includes the step of providing an electrode emissive material in the holes of each dynode layer before that dynode layer is sandwiched between spacer layers. The step of providing an electron emissive material in the holes of each dynode layer may include the step of depositing either the electrode emissive material itself or a precursor adapted to form an electron emissive material into the holes of each dynode layer. For example, the step of providing these plural layers may include the step of forming the layers sequentially, one above the other by selectively depositing the materials of the dynode layers and spacer layers.
In accordance with a further aspect of the invention, a microdynode device may be made by a method including the steps of first providing a set of elongated mandrels extending codirectionally with one another and desirably parallel to one another and then depositing an electrically insulating material over the mandrels to form the spacer layers and a second material to form the dynode layers. These materials are deposited in alternating sequence to form a stack including the dynode layers and the spacer layers in alternating sequence. The method further includes the step of removing the mandrels so as to leave elongated microchannels extending through the stack and including holes extending through the various layers. This method may include the step of depositing an electron emissive material for a precursor adapted to form such a material onto the mandrels adjacent the previously deposited layers of the stack before depositing the second material to form a new dynode layer. Thus, the deposited emissive material will form a lining in the holes of the newly formed dynode layer. The second material deposited to form a dynode layer desirably is an electrically conductive material such as a metal. The mandrels may be formed by a molding process as further discussed below.
A method according to a further aspect of the invention is performed by making one or more dual layer structures. Each dual layer structure is made by providing a spacer layer of an electrically insulating first material, forming depressions in a top surface of this spacer layer and then depositing an electrically conductive material on the top surface to form a dynode layer. The depositing step desirably is performed so that the conductive material extends into the depressions on the top surface so as to form hollow conductive via liners in the depressions as part of the dynode layer. An electron emissive layer is provided on the interior walls of the vias liners and holes are formed extending from the depressions through the spacer layer to the bottom surface of the spacer layer. These steps are repeated so as to form a plurality of dual layer structures and thus form a stack of a plurality of spacer layers and dynode layers. Thus, the dynode layer on the top surface of each spacer layer faces the bottom surface of the next higher spacer layer in the stack. The steps are performed so that the holes and via liners form continuous microchannels extending through the stack. The step of providing each spacer layer formed by depositing insulating first material on the top surface of a previously formed dynode layer so as to form a new spacer layer. The remaining steps of forming depressions depositing the conductive material providing the electron emissive layer and forming the holes may be performed on each new spacer layer after the material of that layer is deposited. Thus, the stack continually grows by addition of new layers. The via liners of each dynode layer may be filled temporarily with a sacrificial plug before depositing the insulating first material to form the next higher spacer layer. The step of forming the holes in the various spacer layers may be formed after the stack is formed and after the spacer layers have been deposited by etching the stack so as to form the holes in all or several of the spacer layers in a single operation. To facilitate etching of holes in the spacer layer, the step of providing the electron emissive material in the via liners desirably includes the step of depositing the electron emissive material so that the emissive material does not coat the bottoms of the depressions. Thus, the electron emissive material may be applied by sputtering or other processes which direct the material along directions oblique to the top surface, so that the material is deposited on the interior walls of the vias, but not on the bottom surfaces of the depressions.
Yet another method of making a microdynode device includes the step of forming an insulating spacer layer and providing a dynode layer on a top surface of the insulating layer. The dynode layer is selectively treated in a plurality of spots so as to form a mesh in each spot, with a plurality of nanoscale passages extending through the dynode layer. For example, where the dynode layers are formed from aluminum, the step of selectively treating the dynode layer may include the step of anodizing the aluminum in the spots. Where the dynode layers are formed from silicon the step of selectively treating the dynode layer in the spots may include the step of anisotropically etching the dynode layer in the spots. An electron emissive material is provided on the interior surfaces of the passages holes are formed in the spacer layer in alignment with the spots so that each hole is in communication with a multiplicity of passages. These steps are repeated and a stack including a plurality of spacer layers and a plurality of dynode layers is formed so that the dynode layer on the top surface of each spacer layer faces the bottom surface of the next higher spacer layer and so that the holes and the mesh spots form continuous microchannels extending through the stack, with the mesh of each spot on a particular dynode layer extending across the microchannel. Desirably, the step of providing a spacer layer is performed by providing a layer of a curable material such as a photoimagable polymer and selectively curing this material to leave a plurality of uncured spots extending through the layer of curable material. The selective curing steps and the steps of selectively treating the dynode layers desirably are performed so that the mesh spots in the dynode layers are disposed in alignment with the uncured spots of the spacer layers. The steps of forming the holes in these spacer layers may be performed by removing the uncured material in the spots of each spacer layer after formation of the mesh in the dynode layer atop that spacer layer. The uncured material may be left in place while additional layers are deposited and the uncured material in the spots of several spacer layers may be removed simultaneously as by directing a washing solution through the microchannels.
Yet another method of making a microdynode device includes the step of providing plural layers of silicon having holes therein and having silicon dioxide layers. A plurality of dynode layers of silicon are also provided. These have holes and a layer of electron emissive material in the holes. Each dynode layer has a layer of an electrically conductive material on a top or bottom surface of the dynode layer. The spacer layers and the dynode layers are stacked so that the holes in the layers are aligned with one another and form continuous microchannels and the stacked layers are then bonded to one another as by anodic bonding to form a monolithic structure.
Thus, particularly preferred forms of the present invention provide a fully integrated, very compact, monolithic, high pixel density, imaging electron multiplier with comparable pixel size and spatial resolution to CCD detectors, but with considerably higher sensitivity, improved signal to noise, faster readout, and lower manufacturing cost. This integrated electron multiplier technology will enable a new generation of compact, rugged, high resolution imaging electron detectors that are expected to find widespread applications in scientific instrumentation, medical imaging, document transmission and reproduction, digital video and still cameras, telecommunications and machine vision.
These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description set forth below, taken in conjunction with the accompanying drawings.